Process for the production of mild steel tubes

ABSTRACT

The invention relates to a process for the production of mild steel tubes suitable for heating installations. Coils of the tubes of mild steel are enclosed in a container and introduced into a furnace and there heated without the addition of protective gas up to a temperature between 640* and 900*C. When this temperature is reached wet hydrogen is introduced into the container in the furnace in order to lower the carbon and nitrogen contents of the steel and increase the size of the grains. Finally the container with the tubes is transferred to a cooling pit in order to cool.

United States Patent Lagercrantz et al.

[75] Inventors: Carl Bertil Bengt Lagercrantz; Carl Johan Staffan Finden, both of Virsbo, Sweden [73] Assignee: Virsbo Bruks AB, Virsbo, Sweden [22] Filed: Mar. 1, 1973 [21] Appl. No.: 337,114

Related US. Application Data [63] Continuation of Serv No. 65,671, Aug. 19, 1970,

abandoned.

[30] Foreign Application Priority Data Aug. 25, 1969 Sweden 11746/69 [52] US. Cl. 148/16 [51] Int. Cl. C2ld 3/04, C2ld 1/74 [58] Field of Search 148/12.l, 16, 167,134

[56] References Cited UNITED STATES PATENTS 1,812,320 6/1931 Carrington 148/134 2,133,532 10/1938 Boegehold.... 148/16 2,360,868 10/1944 Gensamer 148/16 2,557,379 6/1951 Hancock et al 148/16 Feb. 18, 1975 10/1953 Burns et al 148/16 2,656,285 3,105,780 10/1963 Low 3,364,083 1/1968 Minck 148/16 OTHER PUBLICATIONS Ipsen et al., Metal Progress, Oct. 1952, pgs. 123-128.

Primary Examiner-C. Lovell Attorney, Agent, or FirmBrumbaugh, Graves, Donohue & Raymond [57] ABSTRACT The invention relates to a process for the production of mild steel tubes suitable for [heating installations. Coils of the tubes of mild steel are enclosed in a container and introduced into a furnace and there heated without the addition of protective gas up to a temperature between 640 and 900C. When this temperature is reached wet hydrogen is introduced into the container in the furnace in order to lower the carbon and nitrogen contents of the steel and increase the size of the grains. Finally the container with the tubes is transferred to a cooling pit in order to cool.

10 Claims, No Drawings PROCESS FOR THE PRODUCTION OF MILD STEEL TUBES This is a continuation, of application Ser. No. 65,671 filed Aug. 19, 1970 now abandoned.

This invention relates to a process for the production of mild steel tubes suitable for installation purposes.

In connection with heating installations the assembly work including cutting, bending, threading and tube part coupling as well as insulating entails most of the costs. In order to reduce these costs one has started to use small copper tubes instead of steel tubes, since thanks to the mild material, copper tubes may be supplied in coils and easily wound up manually, bent and handled in the desired manner.

It has not been possible previously to use this simple method when using steel tubes, since the mildest steel which is commercially available retains a yield point of about 20 kp/mm even after dead annealing.

The main object of the present invention is to produce a steel tube at a low cost, a tube which is so mild that it may practically be treated in the same way as the small copper tubes and which is suitable for heating installations.

This is achieved according to the present invention in such a way that tubes of mild steel enclosed in containers are introduced into a furnace and then heated there without the addition of protective gas up to a temperature above 640C, preferably above 700C and below 900C. preferably below 850C, and that when this temperature is reached wet hydrogen is introduced into the container in the furnace and is permitted to affect the steel tubes in order to lower the carbon and nitrogen contents and increase the size of the grains, the container with the tubes being then transferred to a cooling pit in order to cool.

Thus the effect of this heat treatment method is twofold, on the one hand one has a considerably lower carbon and nitrogen content which normally creates hardeners in the steel, and on the other an increase in the size of the ferrite-grain is obtained. In this way one may obtain steel tubes with a yield point of 8-15 kp/mm It has previously been known how to decarburize wound-up coils of steel strip in a similar manner. In accordance with the known process nitrogen gas was introduced into the closed container holding the strip coils at room-temperature in order to scour all the air. The container was then transferred to a furnace while at the same time wet hydrogen was introduced. The wet hydrogen was continually introduced during the heating period, the warm-keeping period and the cooling period, and its introduction was not discontinued until 200300C had been reached, at which time the container was transferred to a cooling-pit and nitrogen gas was flushed into the container.

This known process for the treatment of thin steel strip may be applied to the treatment of steel tubes, but it implies the rather expensive use of nitrogen gas and wet hydrogen and besides the result is not entirely satisfactory as regards the tubes.

According to the present invention the charge of nitrogen gas is completely eliminated during the heating period which takes place with air admission. When 640C or preferably 700C has been reached wet hy- 6 drogen is introduced and ignited. In normal cases the admixture of hydrogen and air is considered to be extremely dangerous but at the temperatures in question here the combustion is not explosive, instead it goes off quietly with an insignificant increase in volume of about 5l0 percent. The surface oxidation which takes place when the air enters during the heating process is slight and is reduced off after the introduction of the wet hydrogen.

According to the present invention the consumption of nitrogen gas and wet hydrogen is reduced. According to the present invention wet hydrogen is flushed in only during the warm-keeping period (24 hours, preferably 3 hours), whereas the known process implies the flushing of wet hydrogen during both the heating period, the warm-keeping period and the cooling period.

When the known process for the treatment of steel tubes is applied certain phenomena of embrittlement arise which make it impossible to obtain a completely uniform product. The wet hydrogen used according to the known process had a dew-point of 60C. By essentially lowering the moisture content of the hydrogen in accordance with the present invention the embrittlement has been eliminated. By means of trials it has thus been proved suitable to lower the moisture content during the warm-keeping period down to a dew-point of between 30 and 50C, preferably between 35 and 45C, and entirely during the cooling period, i.e. dry hydrogen is introduced during the cooling down to 200-300C. Of these two modifications of the conventional embodiment the latter is considered the most essential.

The hydrogen used is preferably cracked gas, i.e. cracked ammonia, HgN.

The following may serve as an example of the embodiment according to the present invention.

EXAMPLE.

Tubes of mild carbon steel were wound up into coils and placed in a container. The container was closed and provided with the necessary inlets and outlets for gas. The container was placed in a furnace and heated for 1 hour up to a temperature of 800C. During this time no protective gas was added; the coils were heated in the air which was present in the container from the start. When 800C was reached wet hydrogen having a dew-point of 40C was introduced. This temperature and this supply of hydrogen were maintained constant for three hours, when the container was transferred to a cooling pit; the supply of wet hydrogen is discontinued and replaced by the supply of dry hydrogen. The dry hydrogen was introduced into the container for 2 hours, by which time the temperature had fallen to 250C. After flushing with nitrogen gas for 30 minutes the lid of the container was removed and the final cool ing down to room temperature took place with air admission. i

The tubes obtained had a yield point of 10 kp/mm and could easily be bent manually to the shape desired.

They did not show any tendency to embrittlement.

What is claimed is:

l. A process for the production of mild steel tubes comprising:

a. heating steel tubes in the absence of a protective gas in a heating zone containing air to a temperature in the range of from about 640 to about 900C;

b. supplying wet hydrogen having a dew point of between 30 and 50C to said heating zone when the temperature range is reached and maintaining the hydrogen supply in'that temperature range until a substantial lowering of the carbon and nitrogen content of the tubes and an increase in the grain size of the tubes has been effected; and then 0. cooling the tubes.

2. The process of claim 1 in which the tubes are heated to temperature range of from about 700 to 850C.

3. The process of claim 1 in which the wet hydrogen has a dew point of between 35 and 45C.

4. The process of claim 1 in which the wet hydrogen is supplied to the heating zone for a period of from about 2 to 4 hours.

5. The process of claim 1 in which the wet hydrogen is supplied to the heating zone for a period of about 3 hours.

6. The process of claim 1 in which dry hydrogen is supplied to the heating zone during the cooling of the tubes.

7. The process of claim 1 in which dry hydrogen is supplied to the heating zone while the steel tubes are cooled to a temperature in the range of from about 200 to 300C.

8. The process of claim 7 in which air supplied to the heating zone during cooling of the tubes after the temperature drops below about 200C.

9. The process of claim 1 in which the cooling is continued to a temperature in the range of from about 200 to 300C in about 1 /2 hours to 2 /2 hours.

10. The process of claim 1 in which the cooling is continued at a temperature of from about 200 to about 300C in about 2 hours.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATE NT NO. 3,

DATED February 18, 1975 INVENTOR(S) Lagererantz et al.

It is certified that error appears in the above-identified patent and that said Letters Patent arehereby corrected as shown below:

First page, Item [73] "Virsbo Bruks AB" should be Wirsbo P Bruks AB--.;

First page, Item [63] "Aug. 19, 1970" should be -Aug. 20,

Column 1, line 5, "Aug. 19" should be -Aug. 20 ?Column 1, line 56, "thin" should read -the;

Column 2, line 47, "is" should read -was; and

Claim 8, line 1, after "air" insert --is--.

Bigncd and Scaled this Thirty-firsi Day of October 1978 [SEAL] Attest:

DONALD W. BANNER RUTH-C. MASON Arresting 17 c" Commissioner of Patents and Trademarks 

1. A PROCESS FOR THE PRODUCTION OF MILD STEEL TUBES COMPRISING: A. HEATING STEEL TUBES IN THE ABSENCE OF A PROTECTIVE GAS IN A HEATING ZONE CONTAINING AIR TO A TEMPERATURE IN THE RANGE OF FROM ABOUT 640* TO ABOUT 900*C, B. SUPPLYING WET HYDROGEN HAVING A DEW POINT OF BETWEEN 30* AND 50*C TO SAID HEATING ZONE WHEN THE TEMPERATURE RANGE IS REACHED AND MAINTAINING THE HYDROGEN SUPPLY IN THAT TEMPERATURE RANGE UNTIL A SUBSTANTIAL LOWERING OF THE CARBON AND NITROGEN CONTENT OF THE TUBES AND AN INCREASE IN THE GRAIN SIZE OF THE TUBES HAS BEEN EFFECTED, AND THEN C. COOLING THE TUBES.
 2. The process of claim 1 in which the tubes are heated to temperature range of from about 700* to 850*C.
 3. The process of claim 1 in which the wet hydrogen has a dew point of between 35* and 45*C.
 4. The process of claim 1 in which the wet hydrogen is supplied to the heating zone for a period of from about 2 to 4 hours.
 5. The process of claim 1 in which the wet hydrogen is supplied to the heating zone for a period of about 3 hours.
 6. The process of claim 1 in which dry hydrogen is supplied to the heating zone during the cooling of the tubes.
 7. The process of claim 1 in which dry hydrogen is supplied to the heating zone while the steel tubes are cooled to a temperature in the range of from about 200* to 300*C.
 8. The process of claim 7 in which air supplied to the heating zone during cooling of the tubes after the temperature drops below about 200*C.
 9. The process of claim 1 in which the cooling is continued to a temperature in the range of from about 200* to 300*C in about 1 1/2 hours to 2 1/2 hours.
 10. The process of claim 1 in which the cooling is continued at a temperature of from about 200* to about 300*C in about 2 hours. 